This article was originally published on Eos. The publication contributed the article to Space.com Expert Voices: Op-Ed & Insights.
The Earth froze over 717 million years ago. Ice crept down from the poles to the equator, and the dark subglacial oceans suffocated without sunlight to fuel photosynthesis. Earth became an unrecognizable, alien world—a “snowball earth,” where even the water was colder than freezing.
In Nature Communications, reported researchers the first measured ocean temperature from a snowball Earth episode: −15°C ± 7°C. If this number holds true, it will be the coldest measured ocean temperature in Earth’s history.
“We are dealing with salty brine,” said Ross Mitchella geologist at the Department of Geology and Geophysics of the Chinese Academy of Sciences. “That’s exactly what you see in Antarctica today,” he added, except that the snowball Earth’s brine was slightly colder than even the −13 °C salty slush in Antarctica’s ice-covered Lake Vida today.
Past iron
The Sturtian Snowball was a runaway climate disaster that occurred because ice reflects more sunlight than land or water. Ice reflected sunlight, which cooled the planet, which made more ice, which reflected more sunlight, and so on, until the entire world ended up buried under glaciers that could be up to a kilometer thick.
This unusual time left behind unusual rocks: rusty red iron formations that accumulated where continental glaciers met the ice-covered seas. To take the snowball on Earth’s temperature, the team devised a new way to use that iron as a thermometer.
Iron formations accumulate in water that is rich in dissolved iron. Oxygen transforms the easily soluble, greenish “ferrous” form of iron into rust-red “ferrous” iron that sticks. That’s why almost all iron formations are ancient, relics from a time before Earth’s atmosphere began to fill with oxygen about 2.4 billion years ago, or from the more recent Snowball Earth, when the oceans were sealed under ice. Unable to soak up oxygen from the air or from photosynthesis, snowball Earth’s dark, ice-covered seawater depleted of oxygen.
Iron-56 is the most common iron isotope, but lighter iron-54 rusts more easily. So when iron rusts in the ocean, the remaining dissolved iron is enriched in the heavier isotope. Over many cycles of limited, partial rusting—such as occurred on the anoxic Archean Earth—this enrichment grows, which is why ancient iron formations contain isotopically very heavy iron compared to iron minerals that formed after Earth’s atmosphere and oceans filled with oxygen.
Snowball Earth’s iron is also heavy, even more so than iron formations from the distant, pre-oxygen past. The researchers realized that temperature could be the explanation: Iron minerals that form in cold water end up istopically heavier. We don’t know exactly how hot it was when the ancient iron formations accumulated, but it was probably warmer than under Snowball Earth, when the glaciers reached the equator. Using a previous estimate of 25°C for the temperature of Archean seawater, the team calculated that the water that formed the snowball Earth’s iron formations would likely have been 40°C colder.
“It’s a very interesting, new way to get something different out of iron isotope data,” said the geochemist Andy Heard from the Woods Hole Oceanographic Institution, who were not involved in the study. “It’s a funny, backwards situation to be in where you’re using even older rocks as a baseline to understand something that formed 700 million years ago.”
Partly because of the backwardness, Heard believes the study is best interpreted qualitatively as strong evidence that seawater was very cold, but perhaps not exactly -15°C.
The team also analyzed isotopes of strontium and barium to determine that the snowball of Earth’s seawater was up to 4 times saltier than the modern ocean. Jochen Brocks from the Australian National University, who was not involved in the study, said the researchers’ results are consistent with his own salinity analysis of snowball soil sediments from Australia based on a different method. These rocks formed in a brine that Brocks believes was salty enough to reach -7°C before freezing. Another group reaching a similar conclusion using different methods makes the extreme scenario sound much more plausible, he said.
“It was really cool to get that extra confirmation that it was actually very, very cold,” he said.
Read original article at EOS.org.
